The petroleum industry has often sought new integration opportunities for its refinery products with other processes. One of the areas of interest concerns refinery gases that are currently used as a fuel. In addition, refineries are processing heavier crude oils and sulfur specifications for both diesel and gasoline products are becoming more stringent. Hydrogen can be used within hydrotreaters to remove sulfur, oxygen, and nitrogen and also within hydrocrackers to produce lighter and more paraffinic oils. Consequently, refineries are looking for low cost sources of hydrogen.
While the refinery gases are a potential source of hydrogen, many refinery gas streams are not used either for their hydrogen content or to generate hydrogen through known reforming techniques due to a variety of economic and practical reasons. For instance, the economics for separating hydrogen from refinery gases that contain less than about 30% hydrogen are generally unfavorable. Generally, the hydrogen concentration within such refinery off-gases is too low for the hydrogen to be economically recovered using current available separation technologies.
Several processes are known in the art for the separation and recovery of hydrogen from hydrogen-hydrocarbon feed gas streams. Among these are the following:
U.S. Pat. No. 3,838,553 (Doherty) describes a combination pressure swing adsorption (PSA) and cryogenic process to recover a light gas, especially hydrogen or helium, at high purity and at high recovery from a multicomponent gaseous mixture. The process described by Doherty utilizes both a low temperature separator unit (LTU) and a pressure swing adsorber with recompression of the regeneration stream from the latter and recycle of the recompressed stream to the LTU. However, Doherty requires upgrading the hydrogen stream to at least 95% hydrogen prior to PSA purification. Also, Doherty teaches the recycling of the PSA tailgas, which is uneconomical for PSAs with reasonable recovery (e.g., higher than 80%) due to high compression costs. Furthermore, Doherty does not consider the recovery of heavy hydrocarbons (ethane and heavier).
U.S. Pat. No. 3,691,779 (Meisler et al.) describes a process consisting of a low temperature refrigeration unit and a PSA for producing a high purity, 97 to 99.9% hydrogen. The hydrogen-rich feed, containing methane, nitrogen, carbon monoxide and traces of argon, oxygen, carbon dioxide and light hydrocarbons, passes through a series of cooling and condensation stages having successively lower temperatures, the lowest being −340° F. (120° R). Hydrogen containing vapors and condensate are separated between cooling stages. The hydrogen-enriched gas is sent for further upgrading to an adsorption system operating between −260 to −320° F. (200 to 140° R). A portion of the upgraded stream is expanded, passed through at least one refrigeration stage to provide refrigeration, and then used for regeneration in the PSA system. Most of the refrigeration is provided by Joule-Thomson expansion of the condensates. Meisler et al. teach the use of a capital intensive system, due to the multiple steps required for refrigeration in order to upgrade the feed to approximately 97% hydrogen before sending to PSA. Meisler et al. also teach very low temperature levels in the cold box, driven mostly by the amount of non-condensable compounds present in the feed and the high purity required for the hydrogen stream.
U.S. Pat. No. 7,041,271 (Drnevich et al.) discloses an integrated method for olefins recovery and hydrogen production from a refinery off-gas. After conventional pretreatment, the refinery gas is separated to obtain a light ends stream containing hydrogen, nitrogen and methane, and a heavy ends stream containing olefins. The light ends stream is mixed with natural gas and subjected to reforming and water gas shift reactions for hydrogen production. The heavy ends can be further processed for olefin recovery, such as ethylene and propylene. Drnevich et al. teach the recovery one light end stream from the refinery off-gas. Also, the present invention does not consider the further processing of C2+hydrocarbon stream for olefin/liquefied petroleum gas (LPG) recovery. Furthermore, Drnevich et al. teach the use of low temperature distillation, membrane, PSA and adsorption-desorption processes as means for light end separation, but partial condensation is not discussed.
U.S. Pat. No. 4,749,393 (Rowles et al.) describes a hybrid gas separation process which recovers both heavy hydrocarbon (C2+, C3+ or C4+) and high purity hydrogen products from a gas containing a relatively low concentration of hydrogen (<40%). The process comprises a warm heat exchanger where the feed (together with recycle from the hydrogen purifier) is cooled to an intermediate temperature to allow the recovery of heavy hydrocarbons, a separator coupled with a dephlegmator (reflux condenser) for enriching the heavy hydrocarbons condensate, and a cold heat exchanger followed by a separator, from which an enriched hydrogen gas is obtained and, after warming to recover the refrigeration, is sent to the hydrogen purifier (e.g., PSA, membrane). An optional turboexpander or compressor (depending on the hydrogen purifier requirements) can be added on the hydrogen-enriched stream. The condensate from the cold end separator, rich in methane (80-85%) is used to generate the refrigeration through Joule-Thomson expansion and then is sent to fuel. Additional refrigeration is generated by Joule-Thomson expansion of the condensate from the warm end separator/dephlegmator. The tailgas from the hydrogen purifier (PSA) is recycled to the feed, which can be uneconomical due to recompression requirements. In addition, the use of the dephlegmator for enriching the hydrocarbon condensate can be expensive.
As will be discussed, the present invention provides a process for recovering valuable products, especially hydrogen, from refinery fuel gases in order to economically and practically produce such products and increase efficiency and lower costs.